Conventionally, the methods of packet scheduling have been under discussion with adaptive modulation selection methods in relation to HSDPA (High Speed Down-link Packet Access) under 3GPP (3rd Generation Partnership Project: Standardization organization for technical specifications for 3rd generation mobile systems”).
A 3GPP technical report, namely “12.3.5 Packet Scheduler” (3GTR V0.1.0), proposes a technique in relation to packet scheduling where signals transmitted and received on wireless channels are seen as packets. According to this method, the CIR (Carrier to Interferer Ratio) or the SIR (Signal to Interferer Ratio) is measured at a mobile station end, and the result is reported to a base station, and at the base station end, the mobile station is assigned its priority in packet transmission based on the scale of the CIR or SIR level. By this priority assignment, after a mobile station with high priority finishes communication, the mobile station of the next priority is able to perform communication.
In addition, the idea of achieving optimum transmission rates by changing the modulation scheme in accordance with measurement results of the CIR level or SIR level is under discussion. By giving priority to communications by those mobile stations of good channel conditions, the overall throughput of the communication system can be enhanced.
Meanwhile, Japanese Laid-Open Patent Application Publication No.HEI8-274756 discloses a prior art technique for changing the transmission symbol rate of the downlink. FIG. 1 shows a configuration of the conventional wireless communication system disclosed in the above publication. Referring to this figure, base station 1 comprises antennas 2 and 9, receiver 3, demodulator 4, signal detector 5, controller 6, modulator 7, and transmitter 8. Mobile station 10 comprises antennas 11 and 18, receiver 12, demodulator 13, estimator 14, controller 15, modulator 16 and transmitter 17.
In base station 1, receiver 3 receives a radio signal transmitted from mobile station 10 and inputs the received signal into demodulator 4. Demodulator 4 demodulates the input signal to the original baseband signal, and inputs this into signal detector 5. From the baseband signal input, signal detector 5 extracts a signal that requests a change of the signal transmission rate (i.e. transmission rate of the downlink signals for when signals are sent from base station 1 to mobile station 10), and then inputs this into controller 6. This signal in request of a change of the signal transmission rate is transmitted from mobile station 10.
In response to the signal transmission rate change request signal extracted in signal detector 5, controller 6 generates a signal transmission rate change signal, and inputs this into modulator 7. Modulator 7 modulates the input, signal transmission rate change signal by a prescribed modulation scheme, and inputs this into transmitter 8. Transmitter 8 amplifies the input, modulation signal and transmits it from antenna 9.
Mobile station 10 receives the signal from base station 1 in receiver 12, which is then demodulated to a baseband signal in demodulator 13. From the baseband signal demodulated in demodulator 13, estimator 14 estimates the propagation path between base station 1 and the mobile station, and inputs the result in controller 15. Based on the input, propagation path estimation result, controller 15 decides whether or not to change the transmission rate of the downlink signals. If a change of the transmission rate is to be made, a change request signal is generated and input into modulator 16. Modulator 16 modulates the input, change request signal and generates the modulation signal, which is then input to transmitter 17. Transmitter 17 amplifies the modulation signal from modulator 16 and transmits it from antenna 18. By the above configuration, the transmission rate of the downlink signals from base station 1 to mobile station 10 is changed.
FIG. 2 is a sequence diagram illustrating the process of changing the transmission rate between mobile station apparatus and base station apparatus under conventional wireless communication system. A unique word (i.e. known pattern) for propagation path estimation is inserted on a given interval in the downlink signals from base station 1 to mobile station 10. Mobile station 10 performs propagation path estimation that estimates the correlation with these unique words. Propagation path estimation utilizes output from a correlator which is not shown and output from a received electric field strength measurer which is not shown. More specifically, the transmission rate is changed taking into account (1) the received electric field strength, (2) I-pattern variance, (3) known pattern detection, and (4) the error rate in the mobile station after reception.
Results of propagation path estimation are reported from mobile station 10 to base station 1 on a regular basis in accordance with the performance of propagation path estimation. Base station 1 receives the propagation path estimation results from mobile station 10 and decides whether or not to change the transmission rate of the downlink signals (i.e. determination for transmission rate variation (42)). If the transmission rate of the downlink signals after the determination result in base station 1 regarding transmission rate change (42) is the same as at present, base station 1 reports only the transmission rate of the downlink signals to mobile station 10 (transmission rate report (43)). If the result of transmission rate variation (42) proves a need to change the transmission rate, base station 1 informs mobile station 10 to the effect that the transmission rate will change and the transmission rate to change to, and the change timing (44).
If a report on the propagation path estimation result (41) from mobile station 10 reaches base station 1 during the time after base station 1 informs mobile station 10 to the effect of a transmission rate change (44) and the signal transmission rate changes, base station 1 dismisses the result (45).
Propagation path estimation is performed on a regular basis in mobile station 10, except immediately after the transmission rate of the downlink signals has changed (46). When a result of propagation path estimation is reported from mobile station 10 after an interval, base station 1 validates this report and determines the transmission rate (42). Then, if the transmission rate proves the same as at present, the transmission rate alone is reported to mobile station 10 (43). When the transmission rate of the downlink signals changes, at this point, mobile station 10 is reported to the effect that the transmission rate will change and the transmission rate to change to, and the change timing (44).
The above prior art provides the following two methods of changing the downlink transmission symbol rate.
(i) Method of FIG. 3
Base station 1 transmits (downlink signals) unique words (known patterns) that are time divided for every prescribed period To and that have varying transmission rates. Mobile station 10 receives the unique words corresponding to the respective To's, determines the maximum receivable transmission rate, and reports the result to base station 1 (uplink). By this means, the transmission rate of the downlink can be set at optimum.
(ii) Method of FIG. 4
During the prescribed period To, base station 1 switches the transmission rate for the transmission rate for the downlink signals from high-speed to low-speed in sequence from prescribed transmission rates. Mobile station 10 performs the parity check of the signals transmitted from base station 1 at rate 4 (rate 4), and, only when they are receivable, reports the transmission rate to base station 1. Upon receiving this report on the transmission rate from mobile station 10, base station 1 switches to this transmission rate. According to this method, reception is at the optimum transmission rate, and this makes it possible to quickly set the fastest transmission rate for the propagation path condition at the time of communication.
Incidentally, the symbol rates of the unique words of varying transmission rates include 10 Msps, 20 Msps, 30 Msps, 40 Msps, 50 Msps, 53.24 Msps, 60 Msps, and 70 Msps.
However, conventional wireless communication systems have the following problems.
(1) Given that there is no one fixed SIR (or CIR) measurement method for individual mobile stations and that the elements that configure the circuit for SIR (or CIR) measurement are inconsistent, a base station is unable to accurately compare the downlink signal reception qualities from the respective mobile stations. Due to this, selecting the modulation scheme in consideration of SIR (CIR) measurement result is mediocre in accuracy, and achieving the communication quality above a certain level is difficult.
(2) The wireless communication system disclosed in Unexamined Japanese Patent Application Publication No.HEI8-274756 changes the symbol rate, so radio bandwidth must always leave some room for when using the optimum speed rate. As a result, when a low-speed rate is selected, the rest of the radio bandwidth becomes a waste (that is, a waste of radio resources).